1. Field of the Invention
The present invention relates to a fan system including a plurality of fans, the fan system being preferably installed in an electronic device.
2. Description of the Related Art
A so-called fan tray, which includes an array of a large number of fans, is typically used in a large-scale communication device or router. In the fan tray, the rotation rates of the fans are controlled in accordance with the temperature of an interior of the device. Because there are a variety of PWM (Pulse Width Modulation) control apparatuses designated specifically for fans on the market, the control of the fans has often been performed through PWM. In PWM control, a PWM signal having a pulse width corresponding to the rotation rate of the fan is inputted to the fan. The PWM signal is generated in a control board provided in the fan tray. In the fan, the PWM signal is smoothed and converted into a DC voltage by an internal circuit. A microcomputer inside the fan performs feedback control on the rotation rate in accordance with the DC voltage.
Here, a smoothing circuit in each fan is made up of a capacitor and a resistor. The DC voltage outputted from the smoothing circuit slightly fluctuates as a result of fluctuation in characteristics of these elements. The fluctuation of the DC voltage results in fluctuation of the rotation rate of the fan. Moreover, characteristics of the smoothing circuits vary between different fans, and even if the plurality of fans are controlled to rotate in harmony at the same rotation rate, the fans do not operate in harmony due to the different fluctuations inherently provided by the capacitor and resistor of the smoothing circuits. Accordingly, beating sounds that are produced by the fluctuations in the rotation rates of the individual fans are increased due to variations in the rotation rates of the fans, to thereby result in an unusual beating sound which may be considered as a problematic noise.
JP-A 9-264564, for example, describes a method of reducing interference between noises produced by a plurality of fans. In the method of JP-A 9-264564, rotation rates of blowers are constantly changed in order to reduce beat sounds. This method, however, requires a complicated control process to be performed. Moreover, in the case where a large number of fans are caused to rotate, a period during which rotation rates of a plurality of fans are close to each other may occur, which still results in a beating noise.